Venturi devices comprise a duct or pipe providing a fluid flow passageway that decreases progressively in cross-sectional area in an "upstream section" to a minimum at a "throat section," and then increases progressively again in a "downstream section." Fluid forced through the venturi device has its flow velocity increased progressively in the upstream section to reach a maximum at the throat, the velocity decreasing again in the downstream section, usually accompanied by a considerable turbulence of the fluid in the downstream section and in the duct- or pipe-work fed from the device. The passage of fluid through the device is accompanied by a pressure drop therein, the value of which is proportional to the amount of energy or power required to pass the fluid therethrough. It is usually one of the main endeavors of designers of these devices to keep this pressure drop as low as possible, so that the device and the apparatus in which it is incorporated will operate at maximum efficiency and minimum external power requirements.
In a typical gas scrubbing device such as a venturi scrubber, a gas cleaning liquid (e.g., water) is injected into an incoming particle- or particulate-laden gas stream at or very close to the entrance to the venturi throat, where the gas cleaning liquid is immediately atomized by the high-velocity gas stream into a spray or mist droplets. This mist or spray has a high probability of coming into physical contact with solid material mixed in with the gas stream. This high probability results chiefly from the difference in velocity between the slower moving mist droplets, typically called "target droplets," and the faster moving gas-borne particulates. This high contact probability is also enhanced by the above-mentioned turbulence in the gas downstream of the throat. The liquid droplets pick up the particulate matter after which the droplets holding the particulate material are removed from the stream and collected. A centrifugal entrainment collector is typically employed for receiving and removing the particulate-laden droplets.
The overall collection efficiency of a venturi scrubber is highly dependent on the throat velocity or pressure drop, the liquid-gas-ratio, the chemical wettability of the particulate, and the energy expended to create the target droplets.
The overall effectiveness of the venturi scrubber is a direct function of the percentage of particulates removed from the incoming particulate-laden gas stream. Since the particulates are captured mainly by attaching themselves to target droplets, a major concern in the design of a venturi scrubber is to allow for maximum probability of interaction between target droplets and the particulate-laden gas stream while the gas stream flows through the throat of the venturi scrubber.
In a conventional venturi scrubber, a particulate bearing carrier gas is caused to accelerate when it is forced to pass through a restriction in the containing ductwork (venturi throat). The static pressure of the slow moving gas stream is converted to velocity pressure in the restriction as the gas velocity increases.
Conventional venturi theory holds that the velocity pressure of the carrying gas stream shears the liquid which is administered into the gas stream into fine droplets. The shearing action comes from the differential in velocity of the gas stream relative to the liquid. The size of the resulting droplet created is related to various venturi scrubber physical parameters such as relative velocity, liquid surface tension and viscosity, carrier gas density and viscosity, gas/liquid temperatures, liquid to gas ratio, and other factors.
The target droplets in most of the mathematical models are assumed to be flowing as individual droplets in the gas stream. The particulates, depending upon their size, either flow along streamlines in the gas flow or move through the carrier gas as dictated by thermophoretic, or diffusiophoretic forces.
Generally speaking, the smaller the droplet and the greater the density of droplets per unit volume, the smaller the particle that can be collected. Smaller droplets, given their radius of curvature and reduced surface tension, are assumed to be easier to penetrate. The size of the droplets created is primarily related to scrubber pressure drop.
Equations were developed to predict this pressure drop from certain known operating parameters. Particulate removal efficiencies, however, always seem to be overly optimistic when using the common mathematical models. In other words, either the "real-world" venturi scrubbers had inherent inefficiencies built into them, or the models were wrong. An implication of the results could be that the mechanism for droplet creation could be improved. Another implication could be that the droplets, once created, were not being used properly.
The conventional venturi theory contends that the shearing action in the restricted throat zone creates the droplets. The target droplets in the free stream zone of the throat (the area at or near the center of the throat) would most closely follow the model, but those at or near the throat wall (where local velocities are lower) would not. When the throat zone ended and the venturi section enlarged, the gases and the "large" liquid droplets in the free stream zone would slow down rapidly but the smaller particulate given their smaller size, would continue on their course. The particulate would impact into the slower target droplet and thereby be captured. The smaller particles (below about 0.3 microns) would exhibit little inertia (given their low mass) and would instead by captured by diffusion or interception.
When clear venturi scrubber models were built, however, it was evident that the gas velocity through the typical venturi scrubber throat varied considerably with throat width. Near the wall, the liquid formed a thin stream of liquid with very few droplets and therefore low efficiency. As one moved towards the higher velocities in the center of the throat (free stream area), greater numbers of smaller droplets were created. It is in this center region that the particulate capture models would seem to truly apply. If one could create a throat having a uniform free stream area, with concentrated zones of very small droplets, the capture should improve and more closely follow the mathematical model.
Over the decades that venturi scrubbers have been in use, widely varying throat designs have been tried in an attempt to maximize the scrubber performance.
Attempts have been made to improve recovery efficiency in a venturi throat by placing an obstruction in the throat zone. U.S. Pat. No. 4,023,942 describes a double diamond insert that separates the gas flow through the throat zone into a pair of diverging, constant cross-section throats. The double diamond insert is adjustable so as to be positionable in selected vertical locations within the venturi passage. A similar type of insert is disclosed in U.S. Pat. Nos. 4,049,399, 4,337,229, 3,957,464 and 3,969,482. One disadvantage of the diamond shape is that it creates skewed paths, thereby increasing the complexity of the gas flow path. The inclined angle of the velocity zone (the throat zone) has further disadvantages. In practice, the water target droplets try to move straight downward, impacting on the movable throat section, and causing excess pressure drop. Also, the inclined angle promotes angled collisions between target droplets and gas-entrained particles. Since maximum kinetic energy transfer occurs when the particle directly impacts the center of the droplet, the angled throat does not promote desired direct impact collisions of target droplets and particles.
U.S. Pat. No. 3,870,082 discloses the placement of a plurality of flat parallel physical barriers within a venturi-type device. Although the barriers traverse the venturi throat section, all of the barriers deliberately extend into both the upstream section and the downstream section. In fact, by using barriers of progressively different lengths, the leading and trailing edges of the barriers themselves form the required upstream and downstream sections. For example, the upstream section in U.S. Pat. No. 3,870,082 encompasses the area defined by the leading edges of plates 27 and 28 that extend beyond the shortest plates 29, as depicted in FIG. 1. In this manner, plates 27 and 28 both form and obstruct the upstream section. Within the venturi throat section, all of the barriers have identical lengths. U.S. Pat. No. 3,870,082 teaches that the preferred spacing between the parallel physical barriers is between 5 to 15 times the maximum size of particle that is to be removed by the device. In an exemplary embodiment, a spacing of 0.016 inches (approximately 1/64 of an inch) is employed. U.S. Pat. No. 3,870,082 further discloses that with the preferred spacings the gas flow between the plates is in the form of two back-to-back turbulent boundary layers between two immediately-adjacent liquid films, as illustrated in FIG. 5 of this patent. As described in U.S. Pat. No. 3,870,082, this effect leads to a very high probability that the particles will be trapped by the liquid film and removed from the air stream. Thus, it should be evident that the parallel plate design, as depicted in FIG. 5 of this patent, is not designed to create or employ any free stream zones. Particle capture is mainly performed by the effects of the back-to-back turbulent boundary layers.
In U.S. Pat. No. 3,870,082, a scrubbing liquid is sprayed out from nozzles 23 or 24. As depicted in FIG. 5 of this patent, the liquid runs down the surfaces of the physical barriers to provide thin films 32 thereon. The thin films of liquid are contacted by a particle-laden gas stream (e.g., furnace exhaust gases) which passes between adjacent barriers as the two back-to-back turbulent boundary layers mentioned above. The thin films 32 of liquid capture the particles and carry them away as the liquid trickles down along the surfaces of the physical barriers.
The physical barriers disclosed in U.S. Pat. No. 3,870,082 are also employed in U.S. Pat. No. 4,000,993.
U.S. Pat. No. 4,140,501 discloses a wet gas scrubber having a multiple-throat venturi section. The venturi section comprises three modular venturi units arranged side-by-side for the flow of the air stream therethrough in parallel. Each module comprises a plurality of vertically disposed rods arranged in a plane transverse to the direction of flow of the air stream in spaced-apart relationship. The rods may be, for example, one-inch pipes spaced apart to leave one inch venturi throats between adjacent pipes. In U.S. Pat. No. 4,140,501, length of the venturi throat will necessarily be limited by the diameter of the rods. The throat lengths in the disclosed embodiment will be very short (e.g., one inch) thereby providing only a very small area for particle impaction and capture. The extremely short throat distance does not permit the liquid particles to have any meaningful residence time within the throat zone. Thus, there can be no meaningful mixing of the incoming gas stream with the liquid particles so as to encourage impaction and collection of particulate matter entrained in the incoming gas stream. The venturi throat in U.S. Pat. No. 4,140,501 is probably more accurately characterized as a narrow orifice contactor, as opposed to a venturi.
U.S. Pat. No. 4,012,469 discloses a wet gas scrubber having a throat region comprising an upper and lower row or tray of tubular scrubber rods. The rods are spaced in relation to each other in the direction of the gas flow and are in a parallel relation with one another. The upper rods are connected to a fluid distribution system and have openings in the tube walls for providing wash liquid. The wash liquid (e.g., water droplets) exits the tube walls countercurrent to the incoming gas flow. The incoming gas with foreign particles and liquid droplets entrained therein passes through several venturis formed between the rods of the upper row and the rods of the lower row, thereby causing an increase in velocity. During the traversal of the gas through these venturis, the water droplets are broken up into smaller sizes depending on the velocity of the gas and the resulting drag forces and intimate contact between the particulate matter and the water droplets is produced. There thus results an agglomeration action in the venturi. In U.S. Pat. No. 4,012,469, the lower row of rods is vertically adjustable so as to, in effect, form adjustable venturis between the two rows of rods. In other words, the lower row of rods move up and down perpendicular to the gas flow. The lower row of rods, however, never rise up between the upper tubes and assume the same location, on the same plane. In operation, the lower tubes act like baffles.
In spite of extensive research and exhaustive attempts to improve the performance of venturi scrubbers, there is still a need for further improvements which increase particle capture without significantly affecting the pressure drop, and thereby the amount of energy required to pass the fluid therethrough. There is also still a need for venturi throat inserts which are simple to fabricate, which do not significantly alter the incoming path flow of gas as it passes therethrough and which promotes direct impact collisions between target droplets and particles. The present invention fills that need.